處理一個完整檔案不分割

► 當有些情況需要一個 map 直接處理完一個完整檔案時(不讓系統進行分割分散處理), 一般作法有兩種
1. 設定參數 mapred.min.split.size 讓其等同文件大小. (簡單,但這不是好方法)
2. 覆寫 FileInputFormat.isSplitable() 傳回值為 false , 然後 job.setInputFormatClass(MaxInputFormat.class) 即可

public class MaxInputFormat extends FileInputFormat<LongWritable, Text> {
    @Override
    protected boolean isSplitable(JobContext context, Path filename) {
        return false;
    }
}

►若需要 整個檔案內容作一筆一次 來處理, 則除需要 isSplitable() 傳回值為 false 外, 另需要變更 RecordReader 的行為模式, 然後改以自訂的RecordReader類別來處理.
InputFormat

public class MaxInputFormat extends FileInputFormat<LongWritable, Text> {
    @Override
    protected boolean isSplitable(JobContext context, Path filename) {
        return false;
    }
    @Override //因想要一次讀入全部資料, 故需替換為自訂讀取類別
    public RecordReader<LongWritable, Text> createRecordReader(InputSplit split, TaskAttemptContext context) throws IOException,InterruptedException {
        return new AllDataRecordReader();
    }
}

自訂讀入行為 AllDataRecordReader

public class AllDataRecordReader extends RecordReader<LongWritable, Text> {
    private FileSplit fileSplit;
    private Configuration conf;
    private Text value = new Text();
    private boolean processed = false;
    @Override
    public void initialize(InputSplit split, TaskAttemptContext context) throws IOException, InterruptedException {
        this.fileSplit = (FileSplit) split;
        this.conf = context.getConfiguration();
    }
    @Override
    public boolean nextKeyValue() throws IOException, InterruptedException {
        if (!processed) {
            byte[] contents = new byte[(int) fileSplit.getLength()];
            Path file = fileSplit.getPath();
            FileSystem fs = file.getFileSystem(conf);
            FSDataInputStream in = null;
            try {
                in = fs.open(file);
                IOUtils.readFully(in, contents, 0, contents.length);//一次讀入全部資料
                value.set(contents, 0, contents.length);
            } finally {
                IOUtils.closeStream(in);
            }
            processed = true;
            return true;
        }
        return false;
    }
    @Override
    public LongWritable getCurrentKey() throws IOException,    InterruptedException {
        return new LongWritable(0);
    }
    @Override
    public Text getCurrentValue() throws IOException,InterruptedException {
        return value;
    }
    @Override
    public float getProgress() throws IOException {
        return processed ? 1.0f : 0.0f;
    }
    @Override
    public void close() throws IOException { }
}

至於 RecordReader<LongWritable, Text> 亦可改為 RecordReader<NullWritable, BytesWritable> 型態以 Bytes 進行資料傳遞,也無需key值. 畢竟一次傳完所有資料有無key都一樣. 若使用 BytesWritable 沒有特別的輸出處理則輸出資料為 31 39 35 30 2d 31 32 2d 30 34 20 37 39 39  十六進制值